1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus that removes a particulate matter (PM) in exhaust gas using a continuous regenerative diesel particulate filter (DPF), whereby the DPF can be forcibly regenerated at an appropriate time.
2. Description of the Related Art
In a diesel engine, combustion is performed in a region with a large air excess ratio λ, and therefore, the diesel engine has such a characteristic that although discharge amounts of carbon monoxide (CO) and hydrocarbon (HC) are small, discharge amounts of nitrogen oxide (NOx) and a PM are large. Therefore, as an exhaust gas purification apparatus that reduces the discharge amount of the PM, an exhaust gas purification apparatus that collects the PM by a DPF provided in an exhaust gas passage has been put to practical use. Since a clogging of the DPF advances with increasing the amount of the collected PM, a forced regeneration processing for suitably burning the PM is indispensable. Recently, research and development regarding a continuous regenerative DPF are progressing. The continuous regenerative DPF continuously oxidizes and removes the PM collected by the DPF at a relatively low temperature utilizing nitrogen dioxide (NO2) generated by a diesel oxidation catalyst (DOC). However, when an exhaust gas temperature is low, the oxidation and removal of the PM utilizing NO2 are insufficient, and therefore, the forced regeneration processing cannot be omitted in practice.
In order to determine the forced regeneration time of the DPF, Japanese Laid-Open Patent Publications Nos. 2006-46247 and 2002-180821 propose a technique of estimating an amount of deposited PM from a pressure loss of the DPF. Furthermore, Japanese Laid-Open Patent Publication No. 2006-2672 proposes a technique of integrating an amount of generated PM obtained from an exhaust air-fuel ratio and an amount of oxidized PM obtained from an exhaust gas temperature and the difference therebetween is regarded as the PM deposited amount. Moreover, Japanese Laid-Open Patent Publication No. 2001-207829 proposes a technique of mapping an amount of discharged PM and an amount of burned PM based on engine operating conditions and estimating the PM deposited amount from them.